Choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of animals.

Choanoflagellates are found globally in aquatic environments, and they are of particular interest to evolutionary biologists studying the origins of multicellularity in animals.

As the proposed sister group to Animalia, choanoflagellates serve as a useful model for reconstructions of the last unicellular ancestor of animals.

[citation needed] Each choanoflagellate has a single flagellum, surrounded by a ring of actin-filled protrusions called microvilli, forming a cylindrical or conical "collar" (choanos in Greek).

[citation needed] In addition to the single apical flagellum surrounded by actin-filled microvilli that characterizes choanoflagellates, the internal organization of organelles in the cytoplasm is constant.

[16] Over 125 extant species of choanoflagellates[12] are known, distributed globally in marine, brackish and freshwater environments from the Arctic to the tropics, occupying both pelagic and benthic zones.

[24] The choanoflagellates feed on bacteria and link otherwise inaccessible forms of carbon to organisms higher in the trophic chain.

A paper released in August 2017 showed that environmental changes, including the presence of certain bacteria, trigger the swarming and subsequent sexual reproduction of choanoflagellates.

[28] The ploidy level is unknown;[29] however, the discovery of both retrotransposons and key genes involved in meiosis[30] previously suggested that they used sexual reproduction as part of their life cycle.

Some choanoflagellates can undergo encystment, which involves the retraction of the flagellum and collar and encasement in an electron dense fibrillar wall.

Analysis of choanoflagellate SiTs shows that they are similar to the SiT-type silicon transporters of diatoms and other silica-forming stramenopiles.

[36] Félix Dujardin, a French biologist interested in protozoan evolution, recorded the morphological similarities of choanoflagellates and sponge choanocytes and proposed the possibility of a close relationship as early as 1841.

[15] Over the past decade, this hypothesized relationship between choanoflagellates and animals has been upheld by independent analyses of multiple unlinked genetic sequences: 18S rDNA, nuclear protein-coding genes, and mitochondrial genomes (Steenkamp, et al., 2006; Burger, et al., 2003;[17] Wainright, et al., 1993).

Collared cells are found in other animal groups, such as ribbon worms,[38] suggesting this was the morphology of their last common ancestor.

[39] Cristidiscoidea Fungi Ichthyosporea Corallochytrea Filasterea Animalia (Metazoa) Choanoflagellatea The choanoflagellates were included in Chrysophyceae until Hibberd, 1975.

[40] Recent molecular phylogenetic reconstruction of the internal relationships of choanoflagellates allows the polarization of character evolution within the clade.

Large fragments of the nuclear SSU and LSU ribosomal RNA, alpha tubulin, and heat-shock protein 90 coding genes were used to resolve the internal relationships and character polarity within choanoflagellates.

[21] Each of the four genes showed similar results independently and analysis of the combined data set (concatenated) along with sequences from other closely related species (animals and fungi) demonstrate that choanoflagellates are strongly supported as monophyletic and confirm their position as the closest known unicellular living relative of animals.

The family Salpingoecidae consists of species whose cells are encased in a firm theca that is visible by both light and electron microscopy.

[21] The mapping of character traits on to this phylogeny indicates that the last common ancestor of choanoflagellates was a marine organism with a differentiated life cycle with sedentary and motile stages.

[21] Choanoflagellates;[8] The genome of Monosiga brevicollis, with 41.6 million base pairs,[12] is similar in size to filamentous fungi and other free-living unicellular eukaryotes, but far smaller than that of typical animals.